Glass sheets are heated for processing such as forming, heat strengthening, or tempering, etc. Such heating is conventionally performed by either electric resistance-type furnaces or by forced convection furnaces. Prior electric resistance-type furnaces are disclosed by U.S. Pat. Nos. 3,934,970 McMaster et al., 3,947,242 McMaster et al., and 3,994,711 McMaster, all of which have lower and upper housing portions that support electric resistance elements that provide radiant heating of glass sheets from below and above at their lower and upper surfaces during conveyance within a heating chamber of the furnace housing. Prior forced convection furnaces are disclosed by the U.S. Pat. Nos. of Kenneth R. Kormanyos: 5,669,954; 5,672,191; 5,735,924; 5,762,677; and 5,792,232, all of which have lower and upper forced convection heaters that provide forced convection heating of conveyed glass sheets from below and above at top and bottom surfaces of the glass sheets.
Electric resistance furnaces for heating glass sheets are less expensive to initially manufacture than forced convection furnaces for heating glass sheets and also can be more easily controlled by less involved control apparatus. In addition, electric resistance furnaces for heating glass sheets also result in a greater percentage of the energy input being transferred into the glass for the heating. However, electrical energy is more expensive than natural gas heating and the radiant heating involved cannot efficiently heat low emission-coated glass. Furthermore, radiant heating cannot uniformly heat glass coated partially with paint bands or reflective coatings used in the automotive industry because the coated area absorb more or less radiant heat than adjacent uncoated areas.
Forced convection furnaces for heating glass sheets are not as efficient as electric resistance furnaces since a certain volume of heated air must be continually removed from the furnace to make up for the volume of combusted gas that is being introduced. However, this loss in efficiency is offset by the lower cost of natural gas as compared to electrical energy such that the operational cost for forced convection furnaces are effectively less than for electric resistance furnaces.